This would effectively mean that the feature would only be usable by larger boards and those who have the ability and knowledge to set up SPF or DKIM for their domain.

This is a shame, because it would be pretty cool. There's different action types - one that could be useful would be "Confirm Email" for email confirmation emails, for example.

So, this does't seem feasible. However, I am going to leave the Suggestion open. We will still consider adding the markup anyway in the future, at least, if there is enough call for it. At least if forum admins want to try and get approved they can, or indeed if the requirements get relaxed, they'd be ready to go.